1. Field of the Invention
The present invention relates to an ink composition, an image forming method, and a print material.
2. Description of the Related Art
As an image recording method of forming an image on a recording medium such as paper based on image data signals, an electrophotographic system, sublimation type and melt type heat transfer systems, an inkjet recording system, and the like are available. Among the ink compositions used in the recording of images by an inkjet system, active energy radiation-curable aqueous inks can be suitably used in the printing of images, pretreatments for imparting print suitability to a recording medium, post-treatment for protection and decoration of printed images, and the like. Furthermore, since active energy radiation-curable aqueous inks contain water as a main component, the inks have many excellent features and possibilities such as excellent safety, and applicability to high-density inkjet recording as a result of a decrease in viscosity.
A significantly large number of commercially available active energy radiation-curable inks contain low molecular weight photoinitiators and low molecular weight co-initiators. However, if these initiators are not incorporated into the polymer matrix after the irradiation of an active energy radiation, the initiators may easily bleed out from the cured product. Thus, from the viewpoint of safety, there are demands for suppression of such bleed-out.
As one of the methods for solving these problems, an increase in the molecular weight of the initiator may be used. However, when a cleavage type initiator is used, there are occasions in which an initiator which has been cleaved and has its molecular weight reduced may remain without reacting with polymerizable compounds. On the other hand, when a Norrish II type initiator which interacts with a second molecule while being in an excited state, and generates a radical in a two-molecule reaction, is used as a photoinitiator after an increase in the molecular weight, since the initiator does not undergo a molecular weight reduction, a cured product in which a low molecular weight product does not remain can be obtained. An example of a composition containing a high molecular weight Norrish II type initiator is the ink composition described in JP 2006-28516 A.